LibraryCompiler does not determine the index (interval and count of index). It just convert your .lib file into .db file.
It is library designer (engineer) responsibility to determine number of indexes. It may be done manually (by tunning some spice simulations) or automatically (some library characterization tools may do it). Number and interval of indexes must provide good correlation between StaticTimingAnalysis (based on .lib file) and spice simulation (based on transistor netlist, which mimic the real chip timing).
In the begining of index set, the interval is smaller, then it is increased. It is because, in case of small output pin capacitance, the cell delay (as well as transition) is not linearly increased with capacitance increasing. So, we need to have more indexes in this area.